U.S. Pat. No. 4,663,725, which is assigned to the same assignee as the present application, discloses the use of microprocessor based refrigeration control in a transport refrigeration system. This patent teaches initiating defrost: (1) on a time cycle, such as once every four hours; (2) on a need for defrost, such as when the temperature of the evaporator coil is below 45 degrees F.; and (3) in response to a manually entered request for defrost. In the defrost cycle of the aforesaid patent, the evaporator fan and compressor motor are turned off and electrical resistance elements are turned on. When the temperature of the evaporator coil rises above 56 degrees F., the defrost cycle is terminated. If the defrost cycle is not ended within 45 minutes, a system fault is generated, the defrost cycle is ended, and cooling is resumed.
It would be desirable, and it is an object of the present invention, to expand upon the defrost operation of the aforesaid patent, including adding desirable functions related to whether defrost should be initiated in response to manual and timed requests for defrost, as well as adding desirable protective and diagnostic functions related to defrost, once the defrost process has been initiated.